He holds nothing but a garden tool (3)
I believe the answer is:
hoe
'a garden tool' is the definition.
(I've seen this before)
'he holds nothing' is the wordplay.
'holds' means one lot of letters goes inside another.
'nothing' becomes 'o' (looks like zero - 0).
'he' going around 'o' is 'HOE'.
'but' acts as a link.
(Other definitions for hoe that I've seen before include "Garden tool with a flat blade" , "Use garden tool" , "Romanians (anag) -- European enclave" , "Tool for loosening soil and scraping up weeds" , "Weed" .)